Stack: Stylized concatenation of data.frames or ffdfs

Stacks rectangular datasets on top of each other, possibly
performing several type coercions along the way. For large datasets,
depends on the ff package. Provides an aggressive version of
ffbase::compact for data that may appear be real-typed but is in fact
int/short/byte. For many purposes plyr::rbind.fill may be more appropriate,
but for some kinds of survey data, the rules here work better.